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The second law of thermodynamics why making a mess is just part of

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physics entropy........[mumbling]

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sure everything is in its place if you spend most of your waking life making [Woman walks around bedroom]

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sure things are organized and properly stored the rest of your life will be so

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much happier ensure you can take this advice too far theoretically but we have

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to do everything we can to fight disorder because the universe is [Girl cleaning bedroom with duster]

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fighting back against us stupid messy universe what does a clean

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bedroom have to do with physics to answer that question let's talk about

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thermodynamics the first law of thermodynamics says that energy can't be

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destroyed or created all the energy in the universe already exists it's not [Man with new energy t-shirt appears]

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going anywhere and no new energy is going to walk through the front door

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the second law of thermodynamics actually has a few different definitions

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or different ways we can understand it have you ever dropped a few ice cubes in [Woman places ice cubes in to glass]

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a glass of soda and then forgotten it on the counter

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I haven't when you come back it's a gross watered-down disaster what

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happened heat transfer habit that's what heat [Woman sad at ice melting]

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moves from the warmer soda into the colder ice cubes making them melt so why

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didn't it work the other way around why didn't we he'd go from the ice cubes

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to the soda sure the ice cubes are super cold but they still have internal energy

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but it would be pretty freaky if you put ice in a soda and the ice somehow got

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Colder yeah it turns out that never happens it's actually impossible it [Woman discussing heat transfer]

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would break the second law of thermodynamics one way to explain the

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second law is to say that heat always flows from a higher temperature system

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to a lower temperature system and it never ever goes the other way around

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that just makes sense it's one of those things that's so obvious you wonder why [Scientist appears in a lab]

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they even had to write it down I'm not judging but it turns out that the second

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law is a little more complicated than that because the second law of

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thermodynamics is really about entropy which is a fancy science word for

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disorder or chaos the best definition of the second law of thermodynamics is as

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follows in all natural processes the total entropy of a system and its [Definition of second law appears]

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surrounding environment either stays the same or increases entropy never

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decreases to put that in normal person speak things always become more

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disorderly if we consider both the system and the environment around it

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let's say it's a wonderful day for fun and we get to spend eight hours [Woman enters messy bedroom]

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organizing the house it doesn't get any better than that does it just a full day [Woman cleaning bedroom]

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of organizing and tidying by the time you're done it's almost as if you can't

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detect any sign of human life at all it's perfect it's wonderful it's what

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was I saying oh right entropy so we just showed that

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there can be less disorder in the universe at least here in our little

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pocket of it right sorry but no, first of all anytime you have that much fun

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you're going to work up a sweat that means heat came off of you and went into [Heat molecules coming off woman's head]

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the air and when you heat molecules up they move around and vibrate and don't

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get all worked up which means they get more chaotic and as you scurry around

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putting things away that also disturbs molecules in the air opening and closing

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dresser drawers putting things on hangers the joyful act of throwing junk [Woman emptying bin]

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away all of that involves friction which means heat which means chaos there is no

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escaping it it's enough to drive you crazy if we don't get crazy because

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crazy is chaos chaos is the enemy keep calm and clean [Woman walking away after emptying trash]

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on another example of this happens when we have a bouncy ball we've all played

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with one of these super balls that bounce like crazy right if you drop a [Man picks up black ball]

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super ball without adding any extra energy to it it'll bounce back up pretty

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high but it won't bounce all the way back up to where it started

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part of that is due to gravity part of that is also due to entropy right before [Boy holding super ball]

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the ball hits the ground it's got a lot of kinetic energy as it hits the ball [Boy drops super ball]

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deforms a little creating elastic potential energy then it snaps back into

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its original shape which is why it bounces back up into the air the ground [Ball bouncing back up into air]

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also deforms a little bit too this deformation makes the molecules all

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jumping so some of the kinetic energy as a ball is transferred into internal

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energy of the ball and the ground internal energy means a higher

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temperature and in fact if you had some thermometer keeping track of the super

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ball you'd see it tick up just a little bit like one or two tenths of a degrees [Boy drops ball]

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maybe the ground temperature would go up a teeny bit too since some of the

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kinetic energy is converted into internal energy the ball loses some [Ball bouncing on the ground]

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oomph on the bounce and you guessed it more chaos is created there's just no way around it

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okay circling back how is entropy related to our first explanation of this

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thermodynamics law remember we said that heat flows from the warmer system to the

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cooler system think of it like this which is more orderly a block of ice or [Bowl of water and ice cubes appear]

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a bowl of water it's the ice without a question molecules and a solid are

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tightly aligned there's no molecular slipping and [Molecules vibrating rapidly]

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sliding like you have in a liquid and in general a colder system has less chaos

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than a warmer one the molecules are moving more slowly they're vibrating

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less their little molecular shoes are neatly stacked up a hot system means [Molecules appear by hot and cold systems]

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chaos galore molecules banging into each other electrons flying around

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willy-nilly shoes never being put away uh-huh it makes me itch just thinking

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about it entropy will always increase or stay the

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same if heat flow out of the cooler system and into a [Ice cube appears in glass of soda]

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warmer system that would mean the colder system would become more orderly that's

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never going to happen another way to think about the second law of [Mustang car appears and girl opens the hood]

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thermodynamics is to pop the hood on your car assuming you don't have an

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all-electric car then your engine has piston which means it depends on heat [Pistons in engine turning]

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which makes it well a heated and an internal combustion engine just like the

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one in this car has Pistons a piston basically hangs out in a hollow cylinder [Piston diagram appears]

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at the top of the cylinder the piston is like a plunger that creates a tight seal

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to keep all the air inside so we've got gas inside the piston just hanging out [Gas molecules inside a piston]

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doing a gas thing when suddenly a heat source appears this makes the molecules

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in the gas get excited and less dense which creates pressure in the piston

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which pushes the piston up which makes the gears of the engine turn which makes

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the wheels turn and what do you know you're driving on the highway and a [Car driving on the highway]

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sensible speed of course five miles under the speed limit is best okay so

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the gas expands great job gas but if this process happens only once that's

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not going to get you very far things have to cool down so the piston can sink

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back down and the whole process can repeat it in a car [Combustion engine appears]

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this happens hundreds of times a minute where does that heat go bingo out of the [Gas appears from out of tail pipe]

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tailpipe any kind of heat engine has to be able to dump heat into what's called

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a reservoir in this case reservoir doesn't need a big lake full of drinking [A giant reservoir appears]

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water it means something big enough to be able to absorb all the heat that the

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engine needs to get rid of in the case of a car that means the heat goes [Car engine starts and gas appears from tail pipe]

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through the tailpipe and out into the atmosphere the atmosphere is big enough

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that heat from a car doesn't have much effect on the overall temperature of

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course when you have a bunch of cars with a bunch of pollution and greenhouse

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gases well that's a topic for another much more depressing video with the [Gas appears from lots of cars on the highway]

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Pistons going up and down we know that force is being applied and things are

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moving which means work is being done but since

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all the heat that's generated is dumped into the exhaust system this process [Piston moving up and down]

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isn't 100% efficient and that brings us up to our third and final way of looking

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at the second law of thermodynamics it's impossible for a heat engine to convert

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heat completely into work without any other

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effect in fact there's a nice and clean equation to go along with this idea the [Efficiency of heat engine equation appears]

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efficiency of a heat engine that's what the epsilon stands for equals the work

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done W divided by the heat that's input that's the Q sub H because heat can't be

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totally converted into work, work will always be less than the heat input and

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efficiency will always be less than one not everything in life is about cars you

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know no matter what your one uncle who's obsessed with hot rods might say here's [Uncle stood beside smart car]

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a basic diagram of how another type of heat engine works we've got a high

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temperature reservoir on the one end that feeds into the engine which

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partially converts the heat to work and it sends the excess heat that wasn't

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converted down the line to the low temperature reservoir let's say this

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engine does five thousand joules of work while producing nine thousand joules of

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heat what's the efficiency of this bad boy we [Woman appears in room discussing heat efficiency]

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just went over the equation for heat engine efficiency but let's make sure we

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know how to actually use it there has to be a difference in temperature from the

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heat source to the cold reservoir otherwise heat wouldn't flow and that

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would leave us with an engine that did a whole lot of nothing

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or maybe something worse than nothing kind of defeats the whole purpose of an

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engine and according to our thermodynamic lawyer the engine doesn't [Lawyer points to second law of thermodynamics]

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convert all of the heat into work so what's left over has to exit the system

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so we've basically got two different kinds of heat here we've got the heat

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that enters the system we call that Q sub H then we've got the heat that

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leaves the system we'll make that Q sub L so what do we know in this situation [Woman discussing two different heats]

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for one thing we know that the heat engine produces 9,000 joules of heat is

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that the heat coming into the engine or leaving the engine that would be our new [Woman appears in room with a vacuum]

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friend Q sub L since the engine is producing it and not taking it in this

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9,000 joules is what the engine is dumping into the reservoir and then

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we've got our five thousand joules of work of course our efficiency equation [Efficiency equation appears]

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tells us that a heat engines efficiency equals the work produced over the heat

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entering the engine we still don't know how much heat is coming in but it's not

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too tricky to figure out after all we know that an engine is going to produce

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two things work that's been converted from heat and heat not converted to work

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so if we add these together we've got our starting heat which means that we can

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rewrite our efficiency equation by swapping at the heat coming into the

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engine for the heat leaving the engine plus the work done now we just have to [Equation appears with numbers]

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pop in our numbers and we're all good five thousand joules divided by fourteen

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thousand joules gives us an efficiency of thirty five point seven percent which

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isn't great I certainly hold myself to a higher [Woman walks up to car with the hood popped]

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standard than that but that's the way it goes with heat engines they're just not

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that great with the whole efficiency thing now let's say we've got an engine

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that takes in sixty four thousand five hundred joules of heat and gives up [Steam engine going by the road]

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fifty three thousand nine hundred joules in exhaust

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what's our efficiency here hmm our equation uses work and the heat input to

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figure this out but we don't have work here that's okay though we can tackle

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this in two different ways first we can find the work by subtracting the heat [Vehicle driving slowly down the road]

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leaving from the heat entering that tells us how much heat was converted

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into work in this case that comes to ten thousand six hundred joules divided that

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by good old Q sub H and we've got an efficiency of 16.4% I mean impressive

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the other way to figure this out is to start with one if an engine was 100

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percent efficient the work would equal the heat coming in so this ratio would

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equal one from that we can subtract the result of the heat leaving the system

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divided by the heat coming in so one minus fifty two thousand nine hundred

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joules over sixty four thousand five hundred joules gives us sixteen point

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four percent efficiency see like the old saying goes there is more than one way [Woman uses flamethrower on stove]

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to clean a stove and of course we always need to remember

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that as a result of all this inefficiency and heat dumping more [Molecules coming out of tail pipe]

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entropy is introduced into the universe there's no getting away from that which

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is why I hate this stupid second law why can't we just make things more orderly

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wouldn't that make the universe a better place no one actually likes chaos do they? [Woman cleaning room]

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everything moving around going crazy no one important many rules people just

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doing whatever they want leaving whatever they want not caring

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about anything joggers in the street there are toilets to be [Woman walks into toilets]

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cleaned young man sometimes I swear I'm the only one who cares about order in

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the universe

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